This invention relates to an AFC circuit arrangement for an electronic tuner using a plurality of varactors as a tuning element, or more in particular to an improved circuit for superimposing an AFC voltage on a tuning voltage thereby to control the oscillation frequency of a local oscillator circuit.
In a conventional electronic tuner, an AFC varactor is included in a local oscillator circuit and impressed with an AFC voltage for compensating the variations of the oscillation frequency. Since the varactor is operated at high frequencies, lead wires thereof must be as short as possible, thus making it difficult to design the location of component parts of the electronic tuner and also assemble those parts. In order to overcome the foregoing disadvantages, there was provided a method of controlling the oscillation frequency of the local oscillator circuit by means of superimposing an AFC voltage on a tuning voltage. However, in such conventional methods, the AFC voltage is superimposed on a power source voltage so that the superimposed voltage may be outputted from a slider terminal of a variable resistor together with the tuning voltage, or the AFC voltage is superimposed on the tuning voltage by means of a resistor matrix. Therefore, the foregoing conventional methods have such disadvantages that since AFC voltage dividing ratios vary with the preset position of the variable resistor, the interchannel deviation of AFC sensitivity is increased even when AFC sensitivity is properly preset, and that the variations in the tuning voltage characteristics of the tuner cause variations in the AFC sensitivity.